The research objectives include studying the behavior of grafts incompatible with respect to weak transplantation antigens in mice and rats. Transplantation procedures will also be employed to help elucidate the mode of action of some coat-color determinants of mice. Specifically, the projects deal with (1) investigating the basis for the privilege of neonatal skin grafts incompatible with respect to skin-specific alloantigens in mice; 3) studies on the abolition of tolerance to these antigens; (4) investigating the genetic basis for the expression of, and response to, H-Y in rats; and (5) elucidating the mode of action of two coat-color mutants (steel(S1) and light (Blt)) in mice via transplantation procedures.